The present invention relates to rotary drums for drying particulate matter and particularly relates to rotary drum dryers having flights for creating a veiling effect across the interior of the drum and dams in the drum for preventing hot gases of combustion from bypassing, and directing such gases into, the veiling particulate matter.
Rotary drum dryers are used in many applications for drying particulate matter. For example, in the asphalt industry, it is common practice to flow aggregate through a rotatable drum having a burner for flowing hot gases of combustion in heat exchange relation with the aggregate to dry the latter. Flights are typically provided along the interior surface of the drum and provide a veiling effect across the drum cross-section. The aggregate veil efficiently absorbs the heat from the hot gases of combustion flowing axially through the drum. Because of the differences in flow and drying characteristics of various aggregates, there is no optimum flight design that can be utilized in all rotary drum dryers. Variations in aggregate gradation for different pavement design mix, as well as the use of recycled asphalt, which does not flow through the veiling portion of the drum, renders the tuning of the flights important to obtain optimum operation. Also, fixed flights produce wide variations in the temperature of the gases exhausted from the drum, rendering operation inefficient and costly.
Adjustable flights have previously been used in rotary dryers and have enabled plant owners to adjust their flighting for optimum veiling conditions with various types of aggregates, recycle rates and production rates. Even with adjustable flights, however, problems occur in attempting to produce a veil through the drying section of the drum which will provide substantially uniform veiling across the drum for optimum heat transfer. One problem in efforts to provide uniform veiling is that the flighting, typically on the downhill side of the drum during drum rotation, cascades less than one-half of the material which cascades from the flighting on the uphill side of the drum. In other words, the flighting on the downhill side of the rotary drum produces less dense veiling than the density of the veiling produced by the flighting on the uphill side of the drum. As a result of this variation in veiling density across the drum, gases from the combustion burner frequently follow the course of least resistance through the drum, i.e., through the veiling having the lesser density adjacent the downhill side of the drum, and therefore bypass the veiling aggregate along the other side of the drum. This also undesirably increases the drum skin temperature on the downhill side of the drum.
Dams have previously been provided within rotary drum dryers to deflect gases passing along the wall surfaces for flow back toward the centerline of the drum to achieve more effective heat transfer with the aggregate. These dams are also helpful in retarding the flow of material through the drum at low production rates and provide a more consistent flight loading at low production rates.
According to the present invention, there is provided a combination of dams and adjustable flights where the dams and adjustable flights cooperate to enable the dam to support the flights and permit their adjustment into selected angular positions. Particularly, the flights are mounted adjacent the interior wall surface of the drum at circumferentially spaced positions thereabout by pivotal mountings which cooperate with a dam at each end of the flight. Each dam includes an annular plate extending along the interior surface of the drum, with apertures along the outer periphery of the plate opening into the interior surface of the drum. The end of each flight has a pin received in an aperture for pivotally securing the flight adjacent the interior wall surface of the drum. The dams and flights have attachments cooperable to pivot the flights into selected adjusted angular positions dependent upon the type of aggregates used and other production parameters. To accomplish this, there is provided, for each flight, an arcuate track or slot in each dam at the opposite ends of the flight. Each slot has a plurality of stops or detent positions. Each flight carries a bracket at its opposite ends and a removable pin projects from each bracket for securement in a selected detented position along the slot. In this manner, the angle of each flight relative to a tangent to the drum at the location of the flight can be adjusted by pivoting the flight and engaging and securing the pins in the selected detent positions along the slots.
By employing this construction, flights may be adjusted angularly depending on the type of aggregate to more closely obtain a uniform veiling effect. The dams prevent the gases from bypassing the veiling aggregate by flow down a side of the drum and forces the gases toward the center of the drum into and through the veiling aggregate. The dams also provide a portion of the support for the angular adjustment mechanism of the flights, the angle of the flights determining the holding capacity thereof and the veiling characteristics.
More particularly, the dam is preferably welded to the interior surface of the drum. Thus, the drum surface, in cooperation with the apertures in the dam, supports the flights. Additionally, it will be appreciated that sections of flights are provided the drum at axially spaced positions therealong, with dams intervening between the flight sections. By providing twice as many detent positions and arcuate slots or tracks in each dam as flights in each axial section of the drum, i.e., on each side of the dam, the rows of flights can be circumferentially staggered relative to one another. This increases the uniformity of the density of the veiling through the drum and enhancing the heat transfer from the combustion gases to the aggregate.
In a preferred embodiment according to the present invention, there is provided a rotary dryer comprising a generally elongated cylindrical drum, and a plurality of flights spaced one from the other about the interior surface of the cylindrical drum with each flight extending therefrom at a predetermined angle relative to a tangent to the drum at the location of the flight. A dam extends about the interior surface of the drum and is located adjacent like ends of the flights at a predetermined axial location along the drum and means cooperable between the flights and the dam are provided for mounting the flights at selected angular positions relative to the tangents to the drum, respectively.
In a further preferred embodiment according to the present invention, there is provided a rotary dryer comprising a generally elongated cylindrical drum, and a pair of sections of flights spaced axially one from the other along the drum, each of the sections having a plurality of flights spaced circumferentially one from the other about the interior surface of the cylindrical drum with each flight extending therefrom at a predetermined angle relative to a tangent to the drum at the location of the flight. A dam extends about the interior surface of the drum and is located between the sections of flights at a predetermined axial location along the drum. Means cooperable between the flights in each section thereof and the dam are provided for mounting the flights at selected angular positions relative to the tangents to the drum.
In a further preferred embodiment according to the present invention, there is provided a rotary dryer for drying aggregate comprising a generally elongated drum having an axis, a plurality of flights spaced one from the other circumferentially about the interior surface of the drum for containing aggregate, a dam extending circumferentially about the interior surface of the drum and means cooperable between the flights and the dam for varying the aggregate holding capacity and veiling characteristics of the flights.
Accordingly, it is a primary object of the present invention to provide a novel and improved rotary drum dryer having adjustable flights and dams cooperable with one another to vary the holding capacity and veiling characteristics of the flights depending on the type of aggregates and other production parameters and thereby, among other things, provide for more efficient heat transfer and eliminate or minimize any tendency of the hot gases of combustion to bypass the aggregate veil.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.